Monday, October 31, 2005

Multi-purpose UPS modifications

So, I've been modifying some UPS units and thought others might like to do it. 120VAC can kill you, so don't try this if you're not prepared or are unsure about what you're doing!

I procured a bunch of old UPS units (mostly like these) from my employer who did not want to maintain them anymore. The batteries were ALL completely dead, but the units themselves were just fine. I suspect that quite a few units like these are in similar circumstances - they don't work, and companies are happy to get rid of them so they can get newer technology (higher current ratings, USB connections, etc.).

Anyway, what I've done is to modify them to accept battery power from an external source. This makes them quite versatile and useful to anyone who uses 12 volt batteries for more than just powering a UPS. I use mine for powering many devices, not the least of which are my radios.

With the modification, my UPS units now keep my batteries charged (they float at 13.8 volts and that is usually adjustable). In the event of a power failure, they use the batteries that are attached to them to provide backup power to devices plugged into them (my computer for example). Attaching multiple or large batteries allows the UPS to provide power for longer than it would with just the internal battery. Most of them allow me to use them just like an inverter (provide 120 V AC power to devices when camping, etc.) when not plugged into utility power.

The APC units that I've been working with will charge batteries at a pretty low rate (I still need to measure with dead and fully charged batteries). They're not fast chargers by any means but they should be able to juice up a small 7 Ah battery in a day or so.

Here are the basic ideas:
  • Plug it in and see if it comes on and will power a device. If this is a new-to-you device, it's nice to know if it works properly except for having a dead battery.
  • Now unplug the thing and get to work. (You got this part without my help right?)
  • Remove the dead battery. On units less than 5-6 years old, there's almost always a battery access panel. It is dead right? Even so, try not to short circuit it just in case the UPS circuitry is the problem and the battery is healthy.
  • Turn the power switch on and fiddle with the buttons (mine will usually let out a dying wail as the capacitors discharge completely).
  • Find a way to get the wires that attached to the battery out through the case so you can connect other batteries to the unit.
  • Remove the existing connectors.
  • Connect an extension wire to the short ones that are left in the unit. I have been working on 400-650 VA UPS units and have found that 12 gauge copper wire (like this) works great. You'll definitely want to keep the runs as short as possible (at max draw, a 650 VA UPS can pull 33 amps at 12 volts). 12 gauge copper wire is rated at 30 amps at 12 volts, so be sure to not overload it if you have a larger UPS that could pull more that that at full load.
  • Drill a hole somewhere in the case.
  • Feed the wire through the hole (with a grommet installed to protect the wire).
  • Close up the case.
  • Attach a connector to the wire. I use 30 amp Anderson Power Pole connectors. Of course, your battery should also have the same connectors on it. It would be good to have a fuse on the battery. Many UPS units have internal fuses (the APC ones I'm using have a 40 amp fuse soldered right onto their main circuit board).
  • Plug in the UPS.
  • Turn it on.
  • Check the voltage at the connector to make sure you got the polarity right and that it is what you are expecting.
  • You may want to unplug the unit from the wall or at least turn the unit off before connecting a battery. You may not need to do this, but the first time you test a newly modified unit, it's probably a good idea. Note that some units will keep the battery charge circuit energized while the UPS is switched OFF.
  • Connect a battery. You will want to use a Sealed Lead Acid (Gell or AGM) battery only as the charging circuitry is designed for them. Using a flooded battery like you would use in a car, boat, etc. will not work as well unless you adjust the voltage of the charger.
  • Plug in the UPS.
  • Turn it on.
  • Check the voltage at the battery to make sure it is what you expect.
So, for not a whole lot of money, you can get an older UPS unit that does all of this:
  • UPS
  • 12 volt Sealed Lead Acid battery charger
  • Inverter
Great reading material on batteries is available at Battery University.


  1. I've tried this simple modifications, but in my opinion if larger battery installed, it need larger charger also. How will you manage that?.

    Have you try to modify "capacity" rather than "backup time"? ie: Lets say 650VA to 1200VA modification. That will be a lot more useful.

  2. Since the battery sits around most of the time anyway, I wasn't concerned with charging it quickly (I have other chargers that can do that). The UPS chargers will maintain a battery in the ready state so in case of a power outage, it is ready to use. I have several that are not providing UPS power backup, they are only used to float charge the batteries.

    An upgrade in the current rating would not be cost effective. It would require modification of many of the components inside the unit.


  3. This is a nice posting . I really like this post .

  4. I use my kids Power wheels batteries hooked up to a special cord to power cell phones, my modem, router, vonage, and cordless phones for hurricane use. Very cost efective since the batteries are already laying around.